Optical measurement devices or Lidars are used to determine the speed and the direction of the wind by the detection of the Doppler effect on a back-scatter signal of a light beam emitted by the measurement device into a medium that it is desired to analyze. The Doppler effect consists of a shift in the frequency of a light wave reflected back by a moving object. The distance between the measurement device and the object, for example a group of particles in motion, defines the type of detection of the Doppler shift which can, depending on the case, be of the coherent type or else of the direct or incoherent type.
These optical measurement devices are also used for measuring the velocity of aircraft relative to the surrounding medium; the optical measurement device is called a ‘laser anemometer’ when used in this application.
In the anemometer application, a detection of the coherent type is used; a beam coming from a source of light radiation, for example a laser, is separated into two beams. A first beam called signal beam is sent into the measurement region and a second beam called reference beam or local oscillator forms a reference for the detection of the Doppler shift.
Aerosols present in the atmosphere back-scatter the light from the signal beam whose frequency undergoes a Doppler shift with respect to that of the incident light. The signal back-scattered by the medium is mixed with the reference beam, and the result of this mixing is sent onto the photosensitive surface of a detector. The difference between the frequency of the back-scattered signal and that of the reference beam is measured in the electrical signal delivered by the detector and from this is deduced a measurement of the Doppler frequency shift then of the velocity of the aircraft with the knowledge that the expression linking these two quantities is the following:Fd=2v/λ
v being the projection onto the aiming axis of the laser of the velocity vector of the aircraft relative to the ambient medium (atmosphere);
λ being the wavelength of the emitted beam.
FIG. 1 shows a flow block diagram of a Doppler frequency optical measurement device that constitutes the prior art for a heterodyne anemometer.
The device in FIG. 1 comprises a laser unit ULAS_A 10 supplying a light beam for illuminating a separation unit USEP_A 20 delivering a signal light beam Fs for illuminating an emission/reception optical signal channel EMIREC 50 and a reference light beam Fr for illuminating an optical coupler MEL 30.
The laser unit ULAS_A comprises a source of radiation and an optical device spatially shaping the radiation coming from the source, producing a light beam. The wavelength λ1 of the light beam emitted by the laser unit ULAS_A is for example 1.55 μm which is a wavelength commonly employed in the field of optical Telecommunications and for which the atmosphere is relatively transparent.
The various components of the laser unit ULAS_A are not shown in FIG. 1.
The emission/reception optical signal channel EMIREC comprises an optical signal amplifier BOOS 53, a separation unit USEP_B 54 and an optical head TOP 55 in series, delivering a power optical signal Sinc focused into the reference medium MILREF 60. The emission/reception optical signal channel EMIREC may also comprise an optical signal frequency shifting device DEF 51, for example an acousto-optical modulator, shifting the frequency of the beam applied to it by around one hundred MegaHertz.
The position and orientation of the beam Sinc emerging from the optical head TOP can be controlled.
The separation unit USEP_B comprises, for example, a polarization separation coupler followed by a bidirectional optical link in series. The various components of the separation unit USEP_B are not shown in FIG. 1.
The optical head TOP captures light rays Sr (light echo) that are back-scattered by the reference medium MILREF in a given direction, these back-scattered light rays Sr possibly being subject to a Doppler frequency shift relative to the frequency of the beam Sinc incident on the medium MILREF.
The back-scattered light rays Sr are captured by the optical head TOP and take the form of a light beam called ‘back-scattered signal beam’ which is transported through the separation unit USEP_B in order to illuminate the optical mixing coupler MEL.
The optical mixing coupler MEL receives the reference light beam Fr coming from the coupler USEP_A at a first input and, at a second input, the back-scattered signal beam, or ‘light echo’, originating from the separation unit USEP_B. The optical mixing coupler MEL performs the mixing of the two optical signals applied to its two inputs, producing a beating effect on the photosensitive surface of a detector of a detection unit UDET 40.
The detection unit UDET comprises a photosensitive detector delivering an electrical signal when a light beam of wavelength λ1 is applied to its sensitive surface, this electrical signal varying at the same frequency as the periodic motion of the beating effect, and a signal processing unit which is supplied with the electrical signal coming from the detector and which performs the detection and the extraction of the Doppler frequency shift. The measurement of the velocity v of the aircraft motion together with the determination of the direction of its motion can be deduced from the measurement of the Doppler frequency shift.
The components of the detection unit UDET are not shown in FIG. 1.
One of the main criteria of merit for the anemometer shown in FIG. 1 is the signal-to-noise ratio (SNR) measured at the output of the detector of the detection unit UDET. The higher the SNR value, the more readily the anemometric measurement is performed, and the study of the value taken by this parameter allows, for example, the choice of the components with which the optical measurement device is equipped to be optimized.
The value of the signal-to-noise ratio SNR is maximized when the photon noise, which constitutes the ultimate limit below which it is impossible to go, dominates the other sources of noise (laser intensity noise, detector dark noise, detector thermal noise). This maximization can be attained by increasing the power of the useful signal.
A domination of the photon noise over the other noise terms can be attained by reducing the dark noise power of the detector, and/or by increasing the power of the signal of the reference optical channel, or ‘local oscillator’, POL arriving at the detector. An increase in the power of the local oscillator POL has the drawback of causing additional costs for the detection unit, and this increase is furthermore still limited by the detector saturation level.
Since the power POL is limited, the invention is based on the idea of an increase in the power of the useful signal by an increase in the power of the back-scattered signal PSR arriving at the detector.